1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount for damping or isolating vibrations based on flows of a non-compressible fluid contained therein. More particularly, the present invention is concerned with such a fluid-filled elastic mount which is capable of exhibiting different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto.
2. Discussion of the Prior Art
A cylindrical elastic mount is known, as a vibration damping member interposed between two members of a vibration system, for flexibly connecting these two members. In this type of elastic mount, an inner sleeve and an outer sleeve which are radially spaced apart from each other by a given radial distance are elastically connected by an elastic body interposed therebetween. The elastic mount is adapted to damp or isolate vibrations which are applied between the inner and outer sleeves primarily in a diametric direction of the sleeves. This type of cylindrical elastic mount can be made relatively compact and small-sized, and can be readily designed for a comparatively reduced amount of relative radial displacement between the inner and outer sleeves upon application of even an excessively large vibrational load. For these reasons, the cylindrical elastic mount has been widely used as an engine mount, a differential gear mount and a suspension bushing for automotive vehicles.
Recently, there has been proposed a so-called fluid-filled cylindrical elastic mount having a plurality of fluid chambers which are formed between the inner and outer sleeves and which communicate with each other through an orifice passage, as disclosed in laid-open Publication Nos. 62-196434 and 63-172035 of unexamined Japanese patent applications, and U.S. Pat. No. 4,690,389. In this fluid-filled elastic mount, a non-compressible fluid filling the fluid chambers is forced to flow therebetween through the orifice passage, based on relative pressure changes in the fluid chambers which occur when a vibrational load is applied between the inner and outer sleeves The fluid-filled elastic mount which damps or isolates the applied vibrations based on the resonance of the fluid mass flow through the orifice passage is more effective than the elastic mount which relies on only the elasticity of the elastic body for damping the vibrations.
In the fluid-filled cylindrical elastic mount of the type indicated above, however, an improvement in the vibration damping/isolating function based on the resonance of the fluid is provided with respect to only the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. For instance, the orifice passage may be tuned to effectively provide a high damping effect with respect to relatively low-frequency vibrations based on the fluid resonance. In this case, the orifice passage operates as if it were substantially closed when the frequency of the input vibration is higher than the tuned frequency of the orifice passage. Accordingly, the elastic mount exhibits an excessively high dynamic spring constant, i.e., considerably lowered vibration isolating or damping effect, with respect to the input vibration having a relatively high frequency.
In view of the above drawback of the fluid-filled cylindrical elastic mount, it is proposed to provide two independently formed orifice passages, one of which is tuned to provide a sufficiently reduced dynamic spring constant so as to isolate the high-frequency vibrations, and the other of which is tuned to provide an excellent damping effect for the low-frequency vibrations.
One of the two above-indicated differently tuned orifice passages is tuned so that the resonance frequency of the fluid flowing therethrough becomes higher than that of the fluid flowing through the other orifice passage, and so that the flow resistance of the fluid in the one orifice passage becomes smaller than that of the fluid in the other orifice passage Therefore, the fluid filling the elastic mount is unlikely to flow through the other orifice passage having relatively high flow resistance of the fluid. Consequently, it is rather difficult to ensure a sufficient amount of the fluid flowing through the other orifice passage so as to effectively damp the low-frequency vibrations.
To solve the above-described problem, it is proposed to provide a control valve for selectively opening and closing the above-indicated one orifice passage tuned to the higher resonance frequency, depending upon the type of the input vibration, as disclosed in laid-open Publication No. 63-176843 of unexamined Japanese patent application. The control valve is operated by a suitable actuator, so that the above-indicated one orifice passage is automatically opened and closed to permit the elastic mount to exhibit different vibration isolating or damping characteristics, depending upon the type of the input vibration.
In the thus constructed fluid-filled cylindrical elastic mount, however, the use of the control valve disposed within the relevant orifice passage, and the actuator disposed within the elastic mount results in increased structural complexity and comparatively large size of the elastic mount. Accordingly, the cost of manufacture of the elastic mount is considerably increased.